1. Segmented high resolution fast neutron spectrometer:
present status, response function
J.N. Abdurashitov, V.N. Gavrin, V.L. Matushko, A.A. Shikhin, I.E.Veretenkin, V.E. Yants
Institute for Nuclear Research, Russian Academy of Sciences, Moscow, Russia
J.S. Nico and A.K. Thompson
National Institute of Standards and Technology Neutron Interactions and Dosimetry Group Gaithersburg, MD USA
XIV-th International School
“PARTICLES and COSMOLOGY”
April 16 – 21, 2007
Baksan Valley, Kabardino-Balkaria, Russian Federation

2. How to obtain high Pulse Height Resolution?
If neutron is captured,
En   Epi
Due to non-linear light yield,
 I(Epi)  I( Epi)
If one detects recoils separately and light yield function is known,
En   Ep(Ii)
Main idea: INSULATED SEGMENTS!

3. Main features of the spectrometer
This is a capture-gated detector
Collection of the scintillations from recoil protons for fast neutrons energy determination
Compensation of the scintillator light yield non-linearity for heavy particles by sectioning of the detector
Distributed Multichannel HV System
Advanced hardware-controlled technique of -rays uncorrelated background and temperature-noise of the PMT’s rejection
Pulse shape registration
High (1 s) resolution dead time control

4. The high pulse height resolution fast neutron detector: crucial points
The ratio H/C as high as possible (~1.1 for plastics, for liquids)
1 recoil proton in each section (>90%)
Light yield as much as possible ( photons/MeV)
Light collection as uniform as possible (<10%) – internal reflection
6Li(n,α)3H+4.8 MeV – the highest energy for massive products
Pulse shape discrimination
Fast triggering (2 coincident sections in less than 50 ns)

5. The Detector – external view

6. The Detector – internal view (three planes of four)

7. View to PMT’s and HV cells

8. Design of the optical section

9. Data Acquisition System. Functional Diagram

10. Data Acquisition System – Apparatus Rack
One NIM crate
One CAMAC crate
Industrial rack-mounted PC
Two-channel PCI-interfaced fast digital oscilloscope
Software-controlled functional generator
Hardware high-resolution software-controlled dead time counter

12. Main Features of the System
SM-255 System Controller
Main Features of the System
Cost effective (<100$/channel)
Fully completed functional device
High density, up to 255 individual HV channels per system module
Absence of HV cables and connectors
High stability of output voltages %
The own dissipated power less then 0.05 W/channel
The control via serial line: RS-232-C
Output voltage and current measurement
Onboard fast output current limitation or tripping.

14. Contact information:
Contact Person: Valery Astakhov, JINR, Dubna, Russia
Phone:
Fax:

15. Linear Summator (JINR development)
Features:
Using of fast integral operational amplifiers
SMT components application
Power supply from HV system bus – no additional cables

16. MC simulation of the detector: design
DGEOM (INR) code down to 50 keV, GEANT 50 to 0
Liquid scintillator NE-213
Density 0.84 g cm-3
CnH2n
Light yield used for recoil protons:
I=0.95Ep – 8.0[1 – exp(– 0.1Ep0.9)]
Sectioned detector
Cylindrical section
Full energy releasing

17. MC simulation of the detector: result
1 – measured response of single large detector (350x350 mm) on 14 MeV
2 – simulated response of single large detector (350x350 mm) on 15 MeV
3 – simulated response of segmented detector (10, 20, 30 and 40 mm right to left)
Choice – 16 segments 25х150 mm, FWHM = 6%,  = (x 0.8 with 6Li)

18. Technique of the detector response measurement on 14.1 MeV neutrons
D-T generator:
Using reaction 3H(d,n)4He, Q= MeV
Neutrons energy is En14.1 MeV
Reaction cross-section max400 mb at Td=120 keV
Rate is 103 neutrons s-1
Distance is 20 cm to detector,  1 m to wall
Threshold 20 mV corresponds to MeV of recoil energy

19. Summary of data set collected under 14.1 MeV neutron irradiation
File
MALU, hits
Source, MeV
Thresh, mV
Live Time, s
Numev
Rate, s-1
Scaler Rate, s-1
085326 1
14.1
100
993
95059
95.7
104
094333 3 20
5747
14114
2.5
258
113337 -
6712
13468
2.0
126

20. Step response of the detector on 14.1 MeV neutrons

21. 14.1 MeV neutrons energy distribution on cosmic muons background (electron scale)

22. Nonlinearity of light-yield of scintillator for recoil protons

23. Correction curve for energy determination of recoil protons

24. 14.1 MeV neutrons energy distribution on cosmic muons background (neutron energy scale)

25. Response of the detector on 14.1 MeV neutrons

26. Present status of the spectrometer
Simulation and design optimization done
Section design done
New 16 sections building done
Box and supports done
32 PMT-85 and HV cells done
DAQ system design done
DAQ software ready (CLI, Win)
Lithium doped scintillator not ready!
Detector mounting done
Calibration of the detector done
Response on 14.1 MeV neutrons is presented at first time!

27. Conclusion All equipment and DAQ software are ready for operation
Have to be done:
Decrease effective neutron energy threshold up to 0.5 MeV – need new low-noise PMT’s
Solve multi-gate problem – discriminators improvement
Precision timing diagram measurement and documentation
Lithium doped scintillator remains very actual!
Graphical interface for DAQ software
Real Time operation (Linux platform?)